Threader unit provided in conventional sewing machines are disposed in the left side proximity of a needle bar and is provided with a threader shaft configured vertically movably and rotatably. At the lower end of the threader shaft, a threader hook, a thread guide for engagement of a needle thread, and a thread holder disposed leftward relative to the thread guide are provided. Further, provided at the threader unit are components such as a rotary mechanism that rotates the threader hook in the horizontal direction, and a vertically moving mechanism that vertically moves the threader shaft in response to the user operation of the control lever.
Thread holder is provided with a plate support, a presser plate disposed on the underside of the plate support, and a coil spring that bias the presser plate upward toward the plate support. Needle thread is held by being clamped between the plate support and the presser plate. At the left side of the faceplate of the sewing machine head, a thread cut element is provided. The thread cut element has a groove for passing the needle thread through it and a cutter for cutting the needle thread situated deep inside the groove. Using the thread cutter, the user is allowed to cut the tip of the needle thread engaged in a threading path to a length suitable for starting the sewing operation to be executed after completing the threading operation.
As a preparatory task in executing the threading operation with the threader unit, the user is to engage the needle thread with a predetermined threading path. To elaborate, the needle thread is drawn from a thread spool serving as a needle thread supply and is initially engaged with a thread tensioner and a thread take-up. Then, the needle thread is engaged with the thread guide portion of the needle bar, a thread guide, and then retained by a thread holder. Finally, the tip of the needle thread is cut by the needle cutting element. In order to hold the needle thread with the thread holder, the user is to take the lengthwise intermediate portion of the needle thread and insert it between the plate support and the presser plate.
In threading the needle thread with the threader to the components of the sewing machine under the above described state, the user is to press down the control lever provided on the side surface of the sewing machine head. Responsively, components such as the threader shaft are lowered relative to the needle bar to a predetermined position. Then, the threader hook is rotated by the rotary mechanism to be passed through the eye of the sewing needle, whereafter the tensed needle thread is seized by the thread guide. Then, when the user cancels the lowering of the control lever, the thread hook rotates in the reverse direction to be moved out of the needle eye. Thus, the needle thread seized by the thread hook is passed through the needle eye. Thereafter, the threader shaft, etc., are lifted to their original preparatory positions.
For reliable seizure of needle thread by the thread guide, the needle thread needs to be retained in a tense state by the threader hook without slacking. This requires the user to insert the needle thread far enough into the space between the plate support and the presser plate to reach the radial inner peripheral side for tight clamping between the plate support and the presser plate by the thread holder.
To meet the above requirement, suggestions have been made to create a space between the plate support and the presser plate by placing the lower portion of a sewing machine cover in engagement with the presser plate and opening the presser plate relative to the plate support against the spring force of a coil spring when assuming the preparatory position during the threading operation carried out by the user. Such configuration facilitates the task of inserting the needle thread between the plate support and the presser plate. Further, under such configuration, when executing the threading operation, in other words, when the thread holder is lowered, the engagement between the lower portion of the sewing machine cover and presser plate is cancelled. Thus, the spring force of the coil spring urges the presser plate to be pressed against the plate support to clamp the needle thread.
The space created between the plate support and the presser plate does indeed facilitate needle thread insertion but at the same time, it increases the susceptibility of thread disengagement. Thus, in the threading operation by the user employing the above suggested configuration, the needle thread engaged with the thread holder may be loosened against user intention when the needle thread is cut by the thread cut element after passing the needle thread between the plate support and the presser plate. The slack may ultimately result in disengagement of the needle thread from the thread holder to result in a failure of the threading operation. Further, since the presser plate is configured to be opened relative to the support plate through engagement with the lower portion of the sewing machine cover, the suggested configuration introduces complexity in the structure and in the shapes of the components.